Aerosol Hygroscopic Properties in Seoul during the KORUS-AQ Pre-Campaign

The prediction of the effects of atmospheric aerosols on human health and climate has been known to have large uncertainty. To reduce the uncertainty about the aerosol effect, many studies have been carried out, but still they are insufficient to cover the whole world. Because Seoul is a megacity located downwind of China, there are abundant anthropogenic sources and transported aerosols in Seoul. KORUS-AQ is initiated as a collaborative field measurement project between Korea and U.S. to investigate formation of ozone and aerosol and interactions between chemistries, transport, and various sources in Seoul. The KORUS-AQ campaign will be held in May to June, 2016, and the pre-campaign for KORUS-AQ was held in May to June, 2015. Here we present the results from the pre-campaign. Submicron aerosol number concentration (N_CN), TSI CPC and size distribution (14 nm < particle diameter < 670 nm), TSI SMPS were measured at Korea Institute of Science and Technology (KIST) in Seoul for about four weeks (18 May – 12 June). CCN number concentration (N_CCN), DMT CCN counter, aerosol hygroscopic growth factor (H-TDMA), and aerosol chemical composition (Aerodyne AMS) were also measured at the same location. The average diurnal variation of N_CN shows that distinct peaks are located at morning rush hours (0600 – 0900) and about 1500, the latter of which indicates that particle formation events frequently occurred in Seoul during the pre-campaign period. However, N_CCN does not show noticeable diurnal variation during this period. According to H-TDMA measurement, the fraction of hygroscopic aerosols varied diurnally with the maximum in the afternoon and the minimum during the morning rush hours. For 100 nm particles and larger, most observations showed a bimodal hygroscopic behavior except in the afternoon (1200 – 1800), when unimodal behavior was predominant. For 50 nm particles and smaller, on the other hand, most observation showed a unimodal behavior except during the rush hours. Aerosol chemical composition and its relationship with CCN activity and hygroscopicity will be analyzed and presented at the conference.